Adding new ScatterOp and GatherOp in VectorExt Dialect

[keshavvinayak01]

Due to existing limitations of the existing vector.transfer_read and vector.transfer_write ops, adding a new GatherOp and ScatterOp in VectorExt is required.

One example of current limitations is the that the standard vector.transfer ops use a permutation_map to describe how dimensions of the vector map to dimensions of the memory. Linalg.generic indexing maps will offer much more flexibility and expressibility.

Also, VectorExt has a "better" mask representation where the mask indexing is part of the op definition as an indexing map, transfer_read/write models it as the vector shape read/written from the slice.

cc @Groverkss